The presence of sulphur compounds in water is usually an unacceptable factor. In the case of sulphate, sulphite and thiosulphate, the principal drawbacks are attack on the sewer, eutrophication and silting. In addition, heavy metals, which are particularly undesired because of their toxic properties, are frequently also present in water containing a large amount of sulphur compounds. One type of effluent in which sulphur compounds, in particular sulphite, are a constituent which is difficult to remove is the wash water from flue gas treatment plants. The flue gases from power stations and waste incinerators cause extensive pollution of the environment due to the presence of acidifying sulphur dioxide (SO.sub.2). The harmful effects of acidification on forests, water, buildings etc. are generally known. Other types of effluents containing sulphur compounds are those originating from the printing industry, mining industry, and paper, rubber, leather and viscose industry.
In broad terms two types of method are available for the removal of sulphur-containing compounds, that is to say physicochemical methods and biological methods.
The physicochemical treatment methods include precipitation, ion exchange and membrane filtration (electrodialysis and reverse osmosis). Disadvantages of such methods are the high costs and the large stream of waste which results. In the case of flue gas treatment, absorption on lime or ammonia is usually employed. In this case large amounts of gypsum or ammonium sulphate are formed, which may be partly re-used. However, particularly in the case of gypsum the possible applications are becoming ever fewer because the quality demands for gypsum are becoming ever more stringent and the market for gypsum is becoming saturated.
In the case of a biological treatment, sulphate and sulphite and other sulphur compounds are reduced in an anaerobic step to give sulphide, which in turn can be oxidised to elementary sulphur. Such processes are known, for example from International patent application WO 91/16269 and European patent application 451922.
The advantage of such a method is that only small waste streams remain because the sulphur formed can be re-used. However, the disadvantage is that, especially when the effluent contains little organic matter, electron donors have to be added in order to provide sufficient reduction equivalents for the sulphate reducing bacteria (SRB). The most important electron donors are methanol, ethanol, glucose and other saccharides, organic acids, such as acetic, propionic, butyric and lactic acid, hydrogen and carbon monoxide. The use of these electron donors has the effect of substantially increasing the cost of this method of removal of sulphur from waste streams.
Organic compounds having two or more carbon atoms are found to decompose under anaerobic conditions to give hydrogen and acetate. The hydrogen can be used as an electron donor for the reduction of sulphate and sulphite and the like, but, under normal conditions, about 50% of the acetate is converted to methane by methane producing bacteria (MPB). Under normal anaerobic conditions, methanol is converted to methane for about 90%. In this case, methane formation has the disadvantages that additional electron donor has to be added (increasing the costs) and that a gas stream contaminated with H.sub.2 S is formed which has to be washed and burnt off in the flare.